A communication satellite's transponder is a series of interconnected units which form a communications channel between receiving and transmitting antennas. Most communication satellites carry dozens of transponders, each transponder having a bandwidth of tens of megahertz. Most transponders operate on a “bent pipe” principle by sending a received signal back to earth with minimal processing, e.g., a transponder might amplify a signal and shift the signal from an uplink frequency to a downlink frequency. Some communication satellites use a “regenerative” transponder whereby a received signal is also demodulated, decoded, re-encoded and modulated aboard the satellite.
Two or more digital video services (such as a channel) may be supported by a single transponder. As each transponder has a fixed capacity, the determination of how many digital video services that a particular transponder supports is based on maximizing the limited resources of the transponder as best possible. To arrive at the number of digital video services a particular transponder should support, one could simply assign an equal, fixed-sized share of the total capacity of the transponder to each service carried by the transponder. For example, if the transponder is to support five services, then each service would be assigned a fixed 20% of the capacity of the transponder. In this fashion, the transponder could support additional services until the proportional share of the transponder capacity assigned to a particular service is less than the minimum amount of transponder capacity required to support the worst case scenario or high water mark use-case of that service.
In practice, the amount of transponder capacity required by any particular single digital video service fluctuates over time. To make better use of the transponder in view of the changing needs of the services it supports, an approach termed “statistical multiplexing” has been used to allocate transponder resources to digital video services. In statistical multiplexing, the capacity of the transponder is dynamically adjusted many times a second based on the complexity of the digital video carried by each channel supported by the transponder. The complexity of the digital video is a measure of how much data it takes to describe how to display the digital video. In this way, when a particular channel requires a larger portion of the transponder's capacity, additional transponder resources can be allocated to that channel from another channel which is not using all of its allocated transponder resources at the moment.